Physiologic jaundice occurs when bilirubin, a breakdown product of the red blood cells, is not detoxified at the appropriate rate. Bilirubin levels in preterm infants are often elevated. This can result in damage to various parts of the brain and can consequently affect a range of behaviors. In this study we wished to determine if the subtle impairment in sensory, neurological, and affective function of low risk preterm infants is associated with physiologic jaundice. Two measures of neonatal physiologic jaundice were correlated with 15 behavioral measures, taken from 37 low-risk black preterm infants tested about one month after their jaundice was cleared. The behavioral measures evaluated visual function, motor function, affect and central nervous system integrity. Higher estimates of bilirubin were associated with poorer visual function, quicker cessation of distress in response to social intervention and longer periods of active sleep. The pattern of the results was similar when the effects of health risks other than jaundice were statistically controlled. The results were similar when jaundice was measured by levels of serum bilirubin rather than by estimates of unbound bilirubin. The results support the hypothesis that some aspects of the diverse adverse sequelae of preterm births are associated with damage to the brain by the neurotoxin bilirubin, even in "physiologic jaundice".